Image display device having a drive transistor with a channel length longer than a channel length of individual switching transistors

ABSTRACT

An image display device includes: a pixel array part formed of first to fourth scanning lines arranged in rows, signal lines arranged in columns, pixel circuits in a matrix connected to the scanning lines and signal lines, and a plurality of power source lines which supplies first to third potentials necessary for the operations of pixel circuit; a signal part which supplies a video signal to the signal lines; and a scanner part which supplies a control signal to the first to fourth scanning lines, and in turn scans the pixel circuit for every row, wherein the pixel circuits include a sampling transistor, a drive transistor, first to third switching transistors, a pixel capacitance, and a light emitting device, and a channel length of the drive transistor is made longer than a channel length of the switching transistors to suppress fluctuations in threshold voltage.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 16/268,986, filed Feb. 6, 2019, which is a Continuation of U.S.patent application Ser. No. 15/820,185, filed Nov. 21, 2017, now U.S.Pat. No. 10,229,638, issued Mar. 12, 2019, which is a Continuation ofU.S. patent application Ser. No. 14/991,573, filed on Jan. 8, 2016, nowU.S. Pat. No. 9,984,621, issued May 29, 2018, which is a Continuation ofU.S. patent application Ser. No. 13/064,195, filed on May 10, 2011, nowU.S. Pat. No. 9,818,340, issued on Nov. 14, 2017, which is aContinuation of U.S. patent application Ser. No. 11/889,144, filed onAug. 9, 2007, now U.S. Pat. No. 7,952,542, issued on May 31, 2011, whichclaims priority from Japanese Patent Application JP 2006-222846 filed inthe Japanese Patent Office on Aug. 18, 2006, the entire contents ofwhich being incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an image display device which drivesthe current of a light emitting device arranged for every pixel fordisplay. More specifically, the invention relates to a so-called activematrix image display device which controls the amount of current carriedthrough a light emitting device such as an organic electroluminescentdevice with an insulating gate field effect transistor arranged inindividual pixel circuits. In addition, the invention relates to anelectronic appliance in which a display device like this is assembledtherein.

2. Description of the Related Art

In image display devices such as a liquid crystal display, a largenumber of liquid crystal pixels are arranged in a matrix, thetransmission intensity or the reflection intensity of incident lights iscontrolled for every pixel depending on image information to bedisplayed, and then an image is displayed. This is the same in anorganic electroluminescent display in which an organicelectroluminescent device is used for pixels, but the organicelectroluminescent device is a self light emitting device different fromthe liquid crystal pixel. On this account, the organicelectroluminescent display has higher image visibility than that of theliquid crystal display, and a back light is unnecessary, havingadvantages such as high response speed. In addition, the brightnesslevel (gray scale) of individual light emitting devices can becontrolled by the value of current carried therethrough. The organicelectroluminescent display is greatly different from a voltage controldisplay such as the liquid crystal display in that the organicelectroluminescent display is a so-called current control display.

As similar to the liquid crystal display, the organic electroluminescentdisplay has the drive modes as the simple matrix mode and the activematrix mode. The former has a simple structure, but has a problem thatit is difficult to implement a display in large size and highdefinition. Thus, nowadays, the active matrix mode displays are beingactively developed. In this mode, current carried through a lightemitting device inside individual pixel circuits is controlled by anactive device (generally a thin film transistor: TFT) disposed insidethe pixel circuit, and this mode is described in JP-A-2003-255856,JP-A-2003-271095, JP-A-2004-133240, JP-A-2004-029791 andJP-A-2004-093682.

SUMMARY OF THE INVENTION

FIG. 11 shows a schematic diagram depicting an exemplary conventionalpixel circuit. The pixel circuit is arranged at the portion at which ascanning line in rows which supplies control signals intersects a signalline SL in columns which supplies video signals, and the circuitincludes a sampling transistor Tr1, a pixel capacitance Cs, a drivetransistor Trd, and a light emitting device EL. The sampling transistorTr1 conducts in accordance with the control signal supplied from thescanning line to sample the video signal supplied from the signal lineSL. The pixel capacitance Cs holds the input voltage corresponding tothe sampled video signal. The drive transistor Trd supplies an outputcurrent Ids in a predetermined light emission period in accordance withthe input voltage held at the pixel capacitance Cs. Generally, theoutput current Ids has dependence on the carrier mobility μμ and on thethreshold voltage Vth of the channel region of the drive transistor Trd.The light emitting device EL emits light at the brightness correspondingto the video signal by the output current supplied from the drivetransistor Trd. In addition, in the conventional example shown in FIG.11, the pixel capacitance Cs is connected between a gate G of the drivetransistor Trd and a power source potential Vcc. On the other hand, theanode of the light emitting device EL is connected to a source S of thedrive transistor Trd, and the cathode thereof is grounded. A drain ofthe drive transistor Trd is connected to the power source potential Vcc.

The drive transistor Trd receives the input voltage held at the pixelcapacitance Cs by the gate G, carries the output current Ids between thesource S and the drain D, and carries current to the light emittingdevice EL. Generally, the emission brightness of the light emittingdevice EL is proportional to the amount of current carried. Moreover,the supply amount of the output current of the drive transistor Trd iscontrolled by the gate voltage Vgs, that is, by the input voltagewritten in the pixel capacitance Cs. In the pixel circuit, the inputvoltage to be applied to the gate G of the drive transistor Trd isvaried in accordance with the input video signal to control the amountof current supplied to the light emitting device EL.

Here, the operating characteristic of the drive transistor is expressedby Equation (1) below.Ids=(1/2)μ(W/L)Cox(Vgs−Vth)2  (1)

In the Equation (1) for the transistor characteristics, Ids is the draincurrent carried between the source and the drain, and in the pixelcircuit, it is the output current supplied to the light emitting device.Vgs is the gate voltage that is applied to the gate with reference tothe source, and in the pixel circuit, it is the input voltage describedabove. Vth is the threshold voltage of the transistor. In addition, μ isthe mobility of a semiconductor thin film configuring the channel of thetransistor. In addition to this, W is the channel width, L is thechannel length, and Cox is the gate capacitance. As apparent fromEquation (1) for the transistor characteristics, in the case in whichthe thin film transistor operates in the saturation region, the gatevoltage Vgs exceeds the threshold voltage Vth and increases, and then itis turned onto carry the drain current Ids. In principles, as expressedin Equation (1) for the transistor characteristics, if the gate voltageVgs is constant, the same amount of the drain current Ids is supplied tothe light emitting device at all times. Therefore, when video signals atthe same level are supplied to all the individual pixels configuring thescreen, all the pixels emit light at the same brightness, and theuniformity of the screen is supposed to obtain.

The drive current Ids necessary for the light emitting device formed ofan organic electroluminescent device is as large as a few μA per pixel,an N-channel drive transistor having high mobility μ is desirable inorder to reduce the amplitude of the video signal to intend low powerconsumption. The pixel circuit shown in FIG. 11 is a source followertype in which an N-channel transistor is used for the drive transistorTrd.

However, in the pixel circuit shown in FIG. 11, a problem arises that itis difficult to correct the deterioration of the current-voltagecharacteristic (the I-V characteristics) of the light emitting deviceEL. FIG. 12 shows a graph depicting the I-V characteristics of the lightemitting device EL. The horizontal axis indicates the anode voltage Vaof the light emitting device, and the vertical axis indicates the drivecurrent Ids. In the circuit shown in FIG. 11, the anode potential Va isequal to the source potential of the drive transistor Trd, and the drivecurrent is the drain current Ids carried through the drive transistorTrd. As shown in the graph in FIG. 12, the light emitting device such asan organic electroluminescent device is deteriorated in the I-Vcharacteristics over time, and the characteristic curve is flattenedover time. On this account, in the source follower pixel circuit shownin FIG. 11, in association with the deterioration of the I-Vcharacteristics of the light emitting device, the operating point(source potential) of the drive transistor Trd is changed to cause imagepersistence.

In order to cope with the conventional problem, in recent years, abootstrap pixel circuit is also proposed instead of the source followerpixel circuit. The bootstrap pixel circuit is configured in which thepixel capacitance is connected between the gate G and the source S ofthe drive transistor. In the bootstrap pixel circuit, since the gatevoltage Vgs of the drive transistor Trd is maintained in the pixelcapacitance all the time, even though the anode potential (that is, thesource potential of the drive transistor) is changed due to the aging ofthe I-V characteristics of the light emitting device, the output currentIds corresponding to the gate voltage Vgs can be continuously carriedthrough the light emitting device all the time without being affected bythe I-V characteristics of the light emitting device EL. Thus, eventhough the I-V characteristics of the light emitting device aredeteriorated, the degraded image quality will not occur such as thedeterioration of the screen brightness, or image persistence.

In addition to fluctuations in the I-V characteristics of the lightemitting device, the pixel circuit has variations in the characteristicsof the drive transistor Trd depending on individual pixels. In reality,a thin film transistor (TFT) configured of a semiconductor thin filmsuch as polysilicon has variations in the device characteristics ofindividual devices. Particularly, the threshold voltage Vth is notconstant, and varies in every pixel. As apparent from Equation (1) forthe transistor characteristics described above, when the thresholdvoltage Vth of each of the drive transistors is varied, the draincurrent Ids is varied even though the gate voltage Vgs is constant, andthe brightness is varied in every pixel. Thus, the uniformity of thescreen is impaired. Such a pixel circuit has been previously developedthat incorporates the function of canceling variations in the thresholdvoltage of the drive transistor. For example, it is disclosed inJP-A-2004-133240.

However, even the pixel circuits having the bootstrap function thatcorrects the aging of the I-V characteristics of the light emittingdevice, and the function of canceling variations in the thresholdvoltage of the drive transistor (the threshold voltage correctingfunction) have problems that have to be solved. The threshold voltagecorrecting function can cancel the influence of the threshold voltagethat affects the output current to be supplied to the light emittingdevice. However, the variations in the threshold voltage of the drivetransistor Trd themselves adversely affect the bootstrap function, andimpair the uniformity of the screen, which is desirable to be solved.

According to an embodiment of the invention, variations in the thresholdvoltage of the drive transistor are to be suppressed in order to obtaina screen of high uniformity. The embodiment of the invention is directedto an image display device including: a pixel array part; a scannerpart; and a signal part, wherein the pixel array part is formed of: afirst scanning line, a second scanning line, a third scanning line and afourth scanning line arranged in rows, signal lines arranged in columns,pixel circuits in a matrix connected to the scanning lines and signallines, and a plurality of power source lines which supplies firstpotential, second potential and third potential necessary for theoperations of pixel circuit, the signal part supplies a video signal tothe signal lines, the scanner part supplies a control signal to thefirst scanning line, the second scanning line, the third scanning lineand the fourth scanning line, and in turn scans the pixel circuit forevery row, each of the pixel circuits includes a sampling transistor, adrive transistor, a first switching transistor, a second switchingtransistor, a third switching transistor, a pixel capacitance, and alight emitting device,

the sampling transistor conducts in response to the control signalsupplied from the first scanning line in a predetermined samplingperiod, and samples the signal potential of the video signal suppliedfrom the signal line to the pixel capacitance, the pixel capacitanceapplies input voltage to a gate of the drive transistor in accordancewith the signal potential of sampled video signal, the drive transistorsupplies output current in accordance with the input voltage to thelight emitting device, the light emitting device emits light atbrightness in accordance with the signal potential of the video signalwith the output current supplied from the drive transistor for apredetermined light emission period, the first switching transistorconducts in response to the control signal supplied from the secondscanning line prior to the sampling period, and sets the gate of thedrive transistor to the first potential, the second switching transistorconducts in response to control signal supplied from the third scanningline prior to the sampling period, and sets a source of the drivetransistor to the second potential, and the third switching transistorconducts in response to control signal supplied from the fourth scanningline prior to the sampling period, and connects the drive transistor tothe third potential, whereby it allows voltage corresponding to thethreshold voltage of the drive transistor to be held in the pixelcapacitance to correct the influence of threshold voltage, as well as itagain conducts in response to the control signal supplied from thefourth scanning line in the light emission period, and connects thedrive transistor to the third potential to carry the output currentthrough the light emitting device, in which a channel length of thedrive transistor is made longer than a channel length of the individualswitching transistors to suppress fluctuations in threshold voltage.

In addition, an embodiment of the invention is an image display deviceincluding: a pixel array part; a scanner part; and a signal part,wherein the pixel array part is formed of: a first scanning line, asecond scanning line, a third scanning line and a fourth scanning linearranged in rows, signal lines arranged in columns, pixel circuits in amatrix connected to the scanning lines and signal lines, and a pluralityof power source lines which supplies first potential, second potentialand third potential necessary for the operations of pixel circuit, thesignal part supplies a video signal to the signal lines, the scannerpart supplies a control signal to the first scanning line, the secondscanning line, the third scanning line and the fourth scanning line, andin turn scans the pixel circuit for every row, each of the pixelcircuits includes a sampling transistor, a drive transistor, a firstswitching transistor, a second switching transistor, a third switchingtransistor, a pixel capacitance, and a light emitting device, thesampling transistor conducts in response to the control signal suppliedfrom the first scanning line in a predetermined sampling period, andsamples the signal potential of the video signal supplied from thesignal line to the pixel capacitance, the pixel capacitance appliesinput voltage to a gate of the drive transistor in accordance with thesignal potential of sampled video signal, the drive transistor suppliesoutput current in accordance with the input voltage to the lightemitting device, the light emitting device emits light at brightness inaccordance with the signal potential of the video signal with the outputcurrent supplied from the drive transistor for a predetermined lightemission period, the first switching transistor conducts in response tothe control signal supplied from the second scanning line prior to thesampling period, and sets the gate of the drive transistor to the firstpotential, the second switching transistor conducts in response tocontrol signal supplied from the third scanning line prior to thesampling period, and sets a source of the drive transistor to the secondpotential, and the third switching transistor conducts in response tocontrol signal supplied from the fourth scanning line prior to thesampling period, and connects the drive transistor to the thirdpotential, whereby it allows voltage corresponding to the thresholdvoltage of the drive transistor to be held in the pixel capacitance tocorrect the influence of threshold voltage, as well as it again conductsin response to the control signal supplied from the fourth scanning linein the light emission period, and connects the drive transistor to thethird potential to carry the output current through the light emittingdevice, in which the scanner part is formed on a same substrate as thepixel array part is formed, and a channel length of the drive transistoris made longer than a channel length of the transistors configuring thescanner part to suppress fluctuations in threshold voltage.

Preferably, the channel length of the drive transistor is set to 10 μmor greater. In addition, in the pixel circuit, the channel length of thedrive transistor is set so that during the light emission period, thesource potential of the drive transistor is varied, whereas the inputvoltage applied to the gate of the drive transistor is not varied withreference to the source potential. In addition, in the drive transistor,its output current has dependence on a carrier mobility in a channelregion, and the third switching transistor conducts during the samplingperiod and connects the drive transistor to the third potential, takesthe output current out of the drive transistor while the signalpotential is being sampled, and applies the negative feedback of theoutput current to the pixel capacitance to correct the input voltage,whereby the dependence of the output current on the carrier mobility iscancelled.

According to an embodiment of the invention, the channel length of thedrive transistor is made longer to suppress variations in the thresholdvoltage. With this configuration, variations are reduced in the gain inthe bootstrap operation, and the uniformity of the screen can beimproved significantly. More specifically, the channel length of thedrive transistor is made longer than the channel length of theindividual switching transistors, and the variations in the thresholdvoltage are suppressed. As compared with such a switching transistorthat operates in the linear region as a mere switch, the thresholdvoltage of the drive transistor that operates in the saturation regionin accordance with Equation (1) for the characteristics greatly affectsthe uniformity of the screen, and thus this configuration is effective.In addition, the channel length of the drive transistor is made longerthan the channel length of the transistors configuring the peripheralscanner to suppress the variations in the threshold voltage, which isalso effective. In the case in which the pixel array part and thescanner part are formed on the same substrate by TFT processes, thevariations in the threshold voltage of the drive transistor greatlyaffect the uniformity of the screen, and thus it is effective that thechannel length of the transistor of the scanner part is made muchlonger. In any cases, the channel length of the drive transistor ispreferably set to 10 μm or greater, whereby the range of the variationsin the threshold voltage can be suppressed to such a level that does notaffect the uniformity of the screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram depicting the overall configuration of animage display device according to an embodiment of the invention;

FIG. 2 shows a circuit diagram depicting a pixel formed in the imagedisplay device shown in FIG. 1;

FIG. 3 shows a schematic diagram illustrative of the operation of thepixel circuit shown in FIG. 2;

FIG. 4 shows a timing chart illustrative of the operation of the imagedisplay device shown in FIGS. 2 and 3;

FIG. 5 shows a schematic circuit diagram illustrative of the imagedisplay device according to an embodiment of the invention;

FIG. 6 shows a graph illustrative of the image display device accordingto an embodiment of the invention;

FIG. 7 shows a schematic diagram illustrative of depicting the imagedisplay device according to an embodiment of the invention;

FIG. 8 shows another timing chart illustrative of the operation of theimage display device shown in FIGS. 2 and 3;

FIG. 9 shows a schematic diagram illustrative of the image displaydevice according to an embodiment of the invention;

FIG. 10 shows a graph illustrative of the image display device accordingto an embodiment of the invention;

FIG. 11 shows a circuit diagram depicting an exemplary conventionalpixel circuit;

FIG. 12 shows a graph depicting the current-voltage characteristics of alight emitting device;

FIG. 13 shows a cross section depicting the device configuration of thedisplay device according to an embodiment of the invention;

FIG. 14 shows the module configuration of a plan view depicting thedisplay device according to an embodiment of the invention;

FIG. 15 shows a perspective view depicting a television set having thedisplay device according to an embodiment of the invention;

FIG. 16 shows a perspective view depicting a digital still camera havingthe display device according to an embodiment of the invention;

FIG. 17 shows a perspective view depicting a notebook personal computerhaving the display device according to an embodiment of the invention;

FIG. 18 shows a schematic diagram depicting a mobile terminal devicehaving the display device according to an embodiment of the invention;and

FIG. 19 shows a perspective view depicting a video camera having thedisplay device according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the invention will be described in detailwith reference to the drawings. FIG. 1 shows a block diagram depictingthe overall configuration of an image display device according to anembodiment of the invention. As shown in the drawing, the image displaydevice is basically configured of a pixel array part 1, a scanner partand a signal part. The pixel array part 1 is configured of firstscanning lines WS, second scanning line AZ1, third scanning lines AZ2and fourth scanning lines DS arranged in rows, and signal lines SLarranged in columns, and pixel circuits 2 arranged in a matrix which areconnected to the scanning lines WS, AZ1, AZ2 and DS and the signal linesSL, and a plurality of power source lines which supplies a firstpotential Vss1, a second potential Vss2 and a third potential Vccnecessary for the operation of the individual pixel circuits 2. Thesignal part is configured of a horizontal selector 3, which suppliesvideo signals to the signal lines SL. The scanner part is configured ofa write scanner 4, a drive scanner 5, a first correcting scanner 71 anda second correcting scanner 72, and they supply control signals to thefirst scanning lines WS, the fourth scanning lines DS, the secondscanning lines AZ1 and the third scanning lines AZ2, and in turn scansthe pixel circuits for every row.

FIG. 2 shows a circuit diagram depicting an exemplary configuration ofthe pixel circuit incorporated in the image display device shown inFIG. 1. As shown in the drawing, the pixel circuit 2 includes a samplingtransistor Tr1, a drive transistor Trd, a first switching transistorTr2, a second switching transistor Tr3, a third switching transistorTr4, a pixel capacitance Cs, and a light emitting device EL. Thesampling transistor Tr1 conducts in response to the control signalsupplied from the first scanning line WS in a predetermined samplingperiod, and samples the signal potential of the video signal suppliedfrom the signal line SL into the pixel capacitance Cs. The pixelcapacitance Cs applies the input voltage Vgs to the gate G of the drivetransistor Trd in accordance with the signal potential of the sampledvideo signal. The drive transistor Trd supplies the output current Idscorresponding to the input voltage Vgs to the light emitting device EL.The light emitting device EL emits light at the brightness correspondingto the signal potential of the video signal with the output current Idssupplied from the drive transistor Trd during a predetermined lightemission period.

Prior to the sampling period, the first switching transistor Tr2conducts in response to the control signal supplied from the secondscanning line AZ1, and sets the gate G of the drive transistor Trd tothe first potential Vss1. Prior to the sampling period, the secondswitching transistor Tr3 conducts in response to the control signalsupplied from the third scanning line AZ2, and sets the source S of thedrive transistor Trd to the second potential Vss2. Prior to the samplingperiod, the third switching transistor Tr4 conducts in response to thecontrol signal supplied from the fourth scanning line DS, and connectsthe drive transistor Trd to the third potential Vcc, whereby it allowsthe voltage corresponding to the threshold voltage Vth of the drivetransistor Trd to be held in the pixel capacitance Cs, and corrects theinfluence of the threshold voltage Vth. Moreover, the third switchingtransistor Tr4 again conducts in response to the control signal suppliedfrom the fourth scanning line DS during the light emission period,connects the drive transistor Trd to the third potential Vcc, andcarries the output current Ids through the light emitting device EL.

As apparent from the description above, the pixel circuit 2 isconfigured of five transistors Tr1 to Tr4 and Trd, a single pixelcapacitance Cs and a single light emitting device EL. The transistorsTr1 to Tr3 and Trd are an N-channel polysilicon TFT. Only the transistorTr4 is a P-channel polysilicon TFT. However, an embodiment of theinvention is not restricted thereto, which may freely mix N-channel andP-channel TFTs. For example, the light emitting device EL is a diodeorganic electroluminescent device having an anode and a cathode.However, an embodiment of the invention is not restricted thereto, andthe light emitting device includes all the devices that emit lightdriven with current in general.

FIG. 3 shows a schematic diagram depicting that only the part of thepixel circuit 2 is taken out of the image display device shown in FIG.2. For the sake of clarity, the video signal Vsig sampled by thesampling transistor Tr1, the input voltage Vgs and the output currentIds of the drive transistor Trd, and the capacitance component Coled ofthe light emitting device EL are added. Hereinafter, the operation ofthe pixel circuit 2 will be described with reference to FIG. 3.

FIG. 4 shows a timing chart depicting the pixel circuit shown in FIG. 3.The operation of the pixel circuit shown in FIG. 3 will be describedmore specifically with reference to FIG. 4. FIG. 4 shows the waveformsof the control signals applied to the scanning lines WS, AZ1, AZ2 and DSalong a time base T. For simplifying the notation, the control signalsare denoted by the same signs as the signs of the corresponding scanninglines. Since the transistors Tr1, Tr2 and Tr3 are the N-channel type,they are turned on when the scanning lines WS, AZ1 and AZ2 are at thehigh level, and turned off when at the low level. On the other hand,since the transistor Tr4 is the P-channel type, it is turned off whenthe scanning line DS is at the high level, and turned on when at the lowlevel. In addition, this timing chart also shows the waveforms of thecontrol signals WS, AZ1, AZ2 and DS as well as the potential changes inthe gate G and the source S of the drive transistor Trd.

In the timing chart shown in FIG. 4, the period of timings from T1 to T8is a single field (1 f). During the single field, each row of the pixelarray is in turn scanned for one time. The timing chart shows thewaveforms of the control signals WS, AZ1, AZ2 and DS that are applied toa single row of pixels.

At timing T0 before the field is started, all the control signals WS,AZ1, AZ2 and DS are at the low level. Therefore, the N-channeltransistors Tr1, Tr2 and Tr3 are in the OFF state, whereas only theP-channel transistor Tr4 is in the ON state. Thus, since the drivetransistor Trd is connected to the power source Vcc through thetransistor Tr4 in the ON state, it supplies the output current Ids tothe light emitting device EL in accordance with a predetermined inputvoltage Vgs. Therefore, the light emitting device EL emits light attiming T0. At this time, the input voltage Vgs to be applied to thedrive transistor Trd is expressed by the difference between the gatepotential (G) and the source potential (S).

At timing T1 at which the field is started, the control signal DS isswitched from the low level to the high level. Thus, since thetransistor Tr4 is turned off and the drive transistor Trd is separatedfrom the power source Vcc, the light emission is stopped into the nonemission period. Therefore, when the period enters timing T1, all thetransistors Tr1 to Tr4 are in the OFF state.

Subsequently, when it goes to timing T2, the control signals AZ1 and AZ2are at the high level, and then the switching transistors Tr2 and Tr3are turned on. Consequently, the gate G of the drive transistor Trd isconnected to the reference potential Vss1, and the source S is connectedto the reference potential Vss2. Here, Vss1−Vss2>Vth is satisfied, andVss1−Vss2=Vgs>Vth is made, and after that, Vth correction to be done attiming T3 is prepared. In other words, the period from T2 to T3corresponds to the reset period of the drive transistor Trd. Inaddition, when the threshold voltage of the light emitting device EL isVthEL, it is set to VthEL>Vss2. Thus, a negative bias is applied to thelight emitting device EL, and the state is turned to a so-called reversebias state. The reverse bias state is necessary to successfully performthe Vth correction operation and the mobility correcting operation.

At timing T3, the control signal AZ2 is turned to the low level, andright after this, the control signal DS is also turned to the low level.Thus, the transistor Tr3 is turned off, and the transistor Tr4 is turnedon. Consequently, the drain current Ids is carried through the pixelcapacitance Cs to start the Vth correction operation. At this time, thegate G of the drive transistor Trd is held at Vss1, and the current Idsis carried until the drive transistor Trd is cut off. When it is cutoff, the source potential (S) of the drive transistor Trd is turned toVss1−Vth. At timing T4 after the drain current is cut off, the controlsignal DS is again turned to the high level to cut off the switchingtransistor Tr4. Moreover, the control signal AZ1 is also returned to thelow level, and the switching transistor Tr2 is also turned off.Consequently, Vth is held and fixed to the pixel capacitance Cs. Asdescribed above, the period from T3 to T4 is the period to detect thethreshold voltage Vth of the drive transistor Trd. Here, the detectingperiod from T3 to T4 is called a Vth correction period.

As described above, after Vth correction is performed, the controlsignal WS is switched to the high level at timing T5, and the samplingtransistor Tr1 is turned on to write the video signal Vsig to the pixelcapacitance Cs. The pixel capacitance Cs is smaller enough than theequivalent capacitance Coled of the light emitting device EL.

Consequently, most of the video signal Vsig is written to the pixelcapacitance Cs. Precisely, Vsig−Vss1, the difference of Vsig from Vss 1,is written to the pixel capacitance Cs. Therefore, the voltage Vgsbetween the gate G and the source S of the drive transistor Trd is thelevel (Vsig−Vss1+Vth) that Vth detected and held before is added toVsig−Vss1 sampled at this time. Hereinafter, for simplified description,when it is Vss1=0 V, the voltage Vgs between the gate and the source isVsig+Vth as shown in the timing chart in FIG. 4. The sampling of thevideo signal Vsig is performed until timing T6 at which the controlsignal WS is returned to the low level. In other words, the period fromT5 to T6 corresponds to the sampling period.

Subsequently, the control signal DS is turned to the low level at timingT7, and the switching transistor Tr4 is turned on. Thus, since the drivetransistor Trd is connected to the power source Vcc, the pixel circuitgoes from the non emission period to the light emission period. At thepreceding timing T6, the control signal WS is turned to the low level,and the sampling transistor Tr1 is already turned off. On this account,the gate G of the drive transistor Trd is separated from the signal lineSL. Since the application of the video signal Vsig is released, uponturning on the switching transistor Tr4, the gate potential (G) of thedrive transistor Trd is allowed to rise, and it rises together with thesource potential (S). In addition, in the pixel circuit according to theembodiment, the source of the drive transistor Trd is connected to theanode of the light emitting device EL. On this account, the sourcepotential (S) of the drive transistor Trd is the anode potential Va ofthe light emitting device EL at the same time. The timing chart shown inFIG. 4 also shows the anode potential Va of the light emitting deviceEL. The light emission period is finished at timing T8 before enteringthe subsequent field.

As described above, at timing T7, the gate potential (G) of the drivetransistor Trd is allowed to rise, and the source potential (S) alsorises together. This is the bootstrap operation. During the bootstrapoperation, the voltage Vgs between the gate and the source held in thepixel capacitance Cs maintains the value of (Vsig+Vth). In other words,the bootstrap operation is that the anode potential Va of the lightemitting device EL is allowed to rise while Vgs held in the pixelcapacitance Cs is being maintained at constant. In association with therise of the source potential of the drive transistor (S), that is, therise of the anode potential Va of the light emitting device EL, thereverse bias state of the light emitting device EL is cancelled, andthen the output current Ids flows to actually start the light emissionof the light emitting device EL. The relation between the drain currentIds and the gate voltage Vgs at this time is given as Equation (2) belowby substituting Vsig+Vth into Vgs of Equation (1) for the transistorcharacteristics above.Ids=k·μ(Vgs−Vth)2=K·μ(Vsig)2  (2)

In Equation (2), k=(1/2) (W/L) Cox. It is revealed from Equation (2) forthe characteristics that the term Vth is cancelled and the outputcurrent Ids to be supplied to the light emitting device EL does notdepend on the threshold voltage Vth of the drive transistor Trd.Basically, the drain current Ids is determined by the signal voltageVsig of the video signal. In other words, the light emitting device ELis to emit light at the brightness in accordance with the video signalVsig. In addition to this, the pixel circuit maintains the gate voltageVgs at constant all the time without depending on the source potentialof the drive transistor, that is, the anode potential Va of the lightemitting device. Because of the bootstrap function, the pixel circuitcan stably maintain the screen brightness with no influence of thechanges over time in the I-V characteristics of the light emittingdevice EL.

As described above, even the pixel circuit having the bootstrap functionand the threshold voltage correcting function incorporated therein stillhas a problem to be solved. Prior to describing an embodiment of theinvention, this point will be described with reference to FIG. 5. FIG. 5shows a schematic diagram depicting a single pixel circuit taken out ofthe image display device shown in FIG. 2. Basically, it is the same asthe schematic diagram depicting the pixel circuit shown in FIG. 3, butthe parasitic capacitance Cp is also added for the convenience of thedescription. In the thin film transistor, the parasitic capacitance Cpexists between the gate and the source. Particularly in the pixelcircuit, the parasitic capacitance Cp of the sampling transistor Tr1 orthe switching transistor Tr2 adversely affects the operation of thedrive transistor Trd. More specifically, the parasitic capacitance Cp ofthe transistors Tr1 and Tr2 causes a voltage loss in the bootstrapoperation, and this is involved in the variations in the thresholdvoltage Vth of the drive transistor Trd to cause the brightnessdifference on the screen. In the ideal bootstrap operation, a rise ofthe source potential of the drive transistor is perfectly equal to arise of the gate potential, and the gate voltage Vgs is maintained atconstant. In other words, it is ideal that the bootstrap gain is one.However, actually, a loss occurs in the bootstrap gain because of theinfluence of the parasitic capacitance Cp, and the gate potential has asmaller rise than that of the source potential by that amount. Theproblem here is that a loss in the bootstrap gain is not constant amongthe pixels and it is varied by receiving the influence of the thresholdvoltage of the drive transistor Vth of the individual pixel circuits.Because of the variations in the loss in the bootstrap gain, thebrightness difference occurs among the pixels on the screen to impairthe uniformity.

Further with reference to FIG. 5, the loss in the bootstrap gain will bedescribed in detail. Since the voltage Vgs between the gate and thesource of the drive transistor Trd after the signal voltage Vsig iswritten has been subjected for Vth correction in advance, it is asbelow.Vgs=Vsig−Vss1+VthSubsequently, the sampling transistor Tr1 is turned off, and then theswitching transistor Tr4 is turned on, whereby the drive transistor Trdis connected to the power source Vcc, and the drain current Ids iscarried through the light emitting device EL. At this time, the voltagecorresponding to the drain current Ids is applied to the anode terminalof the light emitting device EL. In the timing chart shown in FIG. 4,the anode voltage (the source voltage of the drive transistor) at thistime is denoted by Va. Thus, when the light emission operation, thesource voltage of the drive transistor rises by Va−Vss1+Vth. On theother hand, since the gate voltage of the drive transistor Trd has theparasitic capacitance Cp, its rise is as below:(Va−Vss1+Vth)×Cs/(Cs+Cp).AS described above, Vgs after the bootstrap operation is expressed byEquation (3) below. In addition, the drain current Ids corresponding tothis Vgs is given by Equation (4) below. However, in Equation (3), Vss1is 0 V for simplified description.

$\begin{matrix}\begin{matrix}{{Vgs} = {{Vsig} - {{Vss}\; 1} + {Vth} -}} \\{( {{Va} - {{Vss}\; 1} + {Vth}} ) \cdot {{Cp}/( {{Cs} + {Cp}} )}} \\{= {{Vsig} + {Vth} - {( {{Va} + {Vth}} ) \cdot {{Cp}/( {{Cs} + {Cp}} )}}}}\end{matrix} & (3) \\{{Ids} = {{k \cdot {\mu( {{Vsig} - {( {{Va} + {Vth}} ) \cdot {{Cp}/( {{Cs} + {Cp}} )}}} )}}2}} & (4)\end{matrix}$

Equation (3) expressing Vgs after bootstrap includes the loss in thebootstrap gain in the third term, having a smaller value than an idealone. In the term of the loss in the bootstrap gain, variables Va and Vthare included as Cp/(Cs+Cp) are coefficients. Generally, since the lightemitting device characteristics are not varied so much among the pixels,variations in the anode potential Va can be ignored. In contrast tothis, the threshold voltage of the drive transistor Vth varies for everypixel. On this account, the term of the loss in the bootstrap gain isvaried for every pixel, and the emission brightness is not uniform amongthe pixels.

Generally, the pixel capacitance Cs is about 200 fF, and the parasiticcapacitance Cp is about 5 fF. Thus, the loss in the bootstrap gainCp/(Cs+Cp) is about 2.5%. On this account, variations of about 2.5% ofVth variations are inevitably included in the light emitting current Idsexpressed in Equation (4). For example, suppose the width between themaximum and the minimum of the Vth variations of the drive transistorTrd is 2 V, the Vgs variations caused by the loss in the bootstrap gainare 50 mV. Here, suppose Vgs=2V in white display in which the screenuniformity is most noticeable, the brightness variations caused by thedifference of 50 mV are about 5%, which can be visually seen. Therefore,the yields of the panel are decreased. Generally, on the fabricationprocesses, the variations in Vth of the drive transistor are distributedin streaks on the screen. Thus, unevenness in streaks occurs on thescreen, causing a reduction in the yields of the panel.

As apparent from the study described above, since the pixel circuit hasthe parasitic capacitance Cp, in principles it is inevitable to have theloss in the bootstrap gain. Suppose the loss in the bootstrap gainuniformly occurs in every pixel, for the entire screen, the brightnessis merely slightly reduced, but image quality is not adversely affected.However, since each of the pixels has the variations in the thresholdvoltage of the drive transistor Vth, the loss in the bootstrap gain isaffected by this and varied among the pixels. Then, in an embodiment ofthe invention, in order to suppress the variations in the loss in thebootstrap gain among the pixels, the Vth variations in the drivetransistor of each of the pixels are suppressed. More specifically, thechannel length L of the drive transistor Trd is designed long thatcontrols the light emitting current Ids of the light emitting device EL,whereby the variations in the threshold voltage of the drive transistorVth can be suppressed. This point will be described with reference to agraph shown in FIG. 6. FIG. 6 shows a graph depicting the relationbetween the length L of the thin film transistor and the Vth variations.The horizontal axis indicates the length L, and the vertical axisindicates the Vth variations (deviations). As apparent from the graph,in the thin film transistor having a polycrystalline silicon film as adevice area, the longer the length L is, the smaller the Vth variationsare. The polycrystalline silicon film is formed of a collection ofsilicon crystal grains, which locally has variations in the distributionof crystal grains. The channel length that is the direction of carryingcurrent is prolonged, whereby the variations in the distribution ofcrystal grains are averaged, and the variations in the threshold voltageare reduced by that amount.

Generally, the drive current necessary for the light emitting devicesuch as an organic electroluminescent device is as large as a few μA perpixel. In order to decrease the amplitude of the input video signal andto intend the low power consumption, the size ratio W/L of the drivetransistor Trd is set as large as possible to enhance the current driveperformance. On the other hand, it is preferable to reduce the pixelsize for a high definition panel, and thus it is also preferable thatthe device area of the drive transistor Trd is small. Therefore, inorder to design the drive transistor Trd to have the size ratio as largeas possible and a smaller device area, generally, it tends to design ashorter length L (the channel length) of the drive transistor Trd.However, in TFTs having low temperature polysilicon in the device area,as shown in FIG. 6, the variations in the Vth characteristics aredeteriorated as the length L of the drive transistor becomes shorter.Suppose the length L of the drive transistor Trd is designed short,because of the variations in the Vth characteristics, even though theinfluence of Vth is removed from the drain current Ids in the operationof canceling Vth, the Vth variations caused by the loss in the bootstrapgain are seen on the screen, causing the deteriorated uniformity. Asapparent from Equation (3) described above, in such a pixel having Vthof the drive transistor Trd greater than that of the surrounding pixels,the brightness is relatively reduced more than that of the surroundingpixel, whereas in a pixel having a smaller Vth than that of thesurrounding pixels, the brightness is relatively high. On this account,unevenness like streaks occurs on the screen.

For the measures against image quality defectives, in an embodiment ofthe invention, the length L of the drive transistor Trd is set long.More specifically, preferably, the length L of the drive transistor Trdis designed to be 10 μm or greater. In the case in which the length L is10 μm or greater, as apparent from the graph shown in FIG. 6, the Vthvariations are within 1 V. Here, in the case in which the loss in thebootstrap gain is 2.5%, the Vgs variations caused by the loss in thebootstrap gain are 25 mV. Suppose the gate voltage Vgs=2V that isapplied to the drive transistor Trd in the white gray scale, thebrightness difference caused by the variations is 2.5% by Equation (3).Generally, since the brightness difference visible in the uniformity inthe white gray scale is 2 to 3%, the length L is designed to be 10 μm orgreater, which allows that the brightness variations caused by the lossin the bootstrap gain can be such level that can hardly visibly seen.Thus, the yields of fabricating the panel can be improved. As apparentfrom the graph shown in FIG. 6, in order to achieve high image quality,it is desirable that the length L of the drive transistor Trd ispreferably longer from 15 μm to 20 μm.

In addition, depending on the scale factor of the pixel circuit, it issometimes difficult to define the length L of the drive transistor Trdby absolute numeric values. Also in this case, it is effective that thechannel length L of the drive transistor Trd is made longer than thechannel length of the switching transistors Tr2, Tr3 and Tr4 and thevariations in the threshold voltage Vth are suppressed. The variationsin the threshold voltage Vth of the drive transistor Trd greatly affectthe brightness through the loss in the bootstrap gain. Then, it iseffective that the channel length L of the drive transistor is setlonger than that of the other switching devices configuring the pixel inorder to suppress the brightness variations. In addition, in the panelusing low temperature polysilicon TFTs, the scanner part is sometimesintegrated and formed as well in the same low temperature polysiliconTFT process in addition to the pixel array part. In this case, it iseffective that the channel length L of the drive transistor Trd includedin each of the pixels of the pixel array part is made longer than thechannel length of the transistor configuring the scanner part and thevariations in the threshold voltage Vth are suppressed. The thresholdvoltage of the drive transistor greatly affects the brightness of thepixel, and it is effective that the channel length is made longer thanthe channel length of the transistor of the scanner part in order toenhance the uniformity of the screen.

FIG. 7 shows a schematic diagram depicting causes that the Vthvariations occur in the drive transistor. As shown in the drawing, adisplay device is formed of a single insulating substrate, which is aflat panel 0. On the panel 0, a pixel array part 1 is formed as well asa write scanner 4, a drive scanner 5, and a horizontal selector 3 inperipherals are integrated and formed. As similar to the pixel arraypart 1 at the center, these peripheral drive parts are integrated andformed of thin film transistors. Generally, the thin film transistor hasa polycrystalline silicon film as the device area. For example, for thepolycrystalline silicon film, an amorphous silicon thin film isdeposited on an insulating substrate, laser beams are applied thereontofor crystallization, and then the film is transformed into a polysiliconthin film. For example, for the application of laser beams, line laserbeams are applied while the films are in turn laminated on the panel 0from above to below, whereby the amorphous silicon film is transformedinto the polycrystalline silicon film. When local fluctuations occur inlaser output in the process of applying laser beams, the differenceoccurs in the crystallizability of the polycrystalline silicon film inthe vertical direction of the panel 0, and this appears as the result ofvariations in the threshold voltage of the thin film transistor.Therefore, typically, the variations in the threshold voltage appear inthe horizontal direction of the panel 0 along the lines of laser beams.In the example shown in the drawing, the threshold voltage Vth is higherin the part of the lines than that of the surroundings, whereas Vth islower in the other lines than that of the surroundings. Since thefluctuations in Vth lead to fluctuations in the bootstrap gain,unevenness appears in the brightness in streaks along the lines. WhenVth becomes smaller than the average, the loss in the bootstrap gain issmall as well. Thus, streaks brighter than the surroundings occur. Incontrast to this, when Vth becomes greater than the average, the loss inthe bootstrap gain is great, causing streaks darker than thesurroundings.

FIG. 8 shows a timing chart depicting another exemplary drive method ofthe image display device shown in FIGS. 2 and 3. The notation similar tothat of the timing chart shown in FIG. 4 is adopted for easierunderstandings. The difference from the drive method shown in FIG. 4 isin that in addition to the threshold voltage correcting operation andthe bootstrap operation, the drive method performs the mobilitycorrecting operation. Hereinafter, the drive method shown in FIG. 8 willbe described in detail. At timing T0 before the field is started, allthe control signals WS, AZ1, AZ2 and DS are at the low level. Therefore,the N-channel transistors Tr1, Tr2 and Tr3 are in the OFF state, whereasonly the P-channel transistor Tr4 is in the ON state. Thus, since thedrive transistor Trd is connected to the power source Vcc through thetransistor Tr4 in the ON state, it supplies the output current Ids tothe light emitting device EL in accordance with a predetermined inputvoltage Vgs. Therefore, the light emitting device EL emits light attiming T0. At this time, the input voltage Vgs to be applied to thedrive transistor Trd is expressed by the difference between the gatepotential (G) and the source potential (S).

At timing T1 at which the field is started, the control signal DS isswitched from the low level to the high level. Thus, since thetransistor Tr4 is turned off and the drive transistor Trd is separatedfrom the power source Vcc, the light emission is stopped into the nonemission period. Therefore, when it enters timing T1, all thetransistors Tr1 to Tr4 are in the OFF state.

Subsequently, when the period goes to timing T2, the control signals AZ1and AZ2 are at the high level, and then the switching transistors Tr2and Tr3 are turned on. Consequently, the gate G of the drive transistorTrd is connected to the reference potential Vss1, and the source S isconnected to the reference potential Vss2. Here, Vss1−Vss2>Vth issatisfied, and Vss1−Vss2=Vgs>Vth is made, and after that, Vth correctionto be done at timing T3 is prepared. In other words, the period from T2to T3 corresponds to the reset period of the drive transistor Trd. Inaddition, when the threshold voltage of the light emitting device EL isVthEL, it is set to VthEL>Vss2. Thus, a negative bias is applied to thelight emitting device EL, and the state is a so-called reverse biasstate. The reverse bias state is necessary to successfully perform theVth correction operation and the mobility correcting operation.

At timing T3, the control signal AZ2 is turned to the low level, andright after this, the control signal DS is also turned to the low level.Thus, the transistor Tr3 is turned off, and the transistor Tr4 is turnedon. Consequently, the drain current Ids is carried through the pixelcapacitance Cs to start the Vth correction operation. At this time, thegate G of the drive transistor Trd is held at Vss1, and the current Idsis carried until the drive transistor Trd is cut off. When it is cutoff, the source potential (S) of the drive transistor Trd is turned toVss1−Vth. At timing T4 after the drain current is cut off, the controlsignal DS is again turned to the high level to cut off the switchingtransistor Tr4. Moreover, the control signal AZ1 is also returned to thelow level, and the switching transistor Tr2 is also turned off.Consequently, Vth is held and fixed to the pixel capacitance Cs. Asdescribed above, the period from T3 to T4 is the period to detect thethreshold voltage Vth of the drive transistor Trd. Here, the detectingperiod from T3 to T4 is called a Vth correction period.

As described above, after Vth correction is performed, the controlsignal WS is switched to the high level at timing T5, and the samplingtransistor Tr1 is turned on to write the video signal Vsig to the pixelcapacitance Cs. The pixel capacitance Cs is smaller enough than theequivalent capacitance Coled of the light emitting device EL.Consequently, most of the video signal Vsig is written to the pixelcapacitance Cs. Precisely, Vsig−Vss1, the difference of Vsig from Vss 1,is written to the pixel capacitance Cs. Therefore, the voltage Vgsbetween the gate G and the source S of the drive transistor Trd is thelevel (Vsig−Vss1+Vth) that Vth detected and held before is added toVsig−Vss1 sampled at this time. Hereinafter, for simplified description,when it is Vss1=0 V, the voltage Vgs between the gate and the source isVsig+Vth as shown in the timing chart in FIG. 4. The sampling of thevideo signal Vsig is performed until timing T7 at which the controlsignal WS is returned to the low level. In other words, the period fromT5 to T7 corresponds to the sampling period.

At timing T6 before timing T7 at which the sampling period is finished,the control signal DS is turned to the low level, and the switchingtransistor Tr4 is turned on. Thus, since the drive transistor Trd isconnected to the power source Vcc, the pixel circuit goes from the nonemission period to the light emission period. As described above, in theperiod from T6 to T7 in which the sampling transistor Tr1 is still inthe ON state and the switching transistor Tr4 is in the ON state, themobility of the drive transistor Trd is corrected. In other words, inthis example, in the period from T6 to T7 in which the latter portion ofthe sampling period is overlapped with the beginning portion of thelight emission period, the mobility is corrected. In addition, in thebeginning of the light emission period in which the mobility iscorrected, since the light emitting device EL is actually in the reversebias state, it does not emit light. In the period from T6 to T7 tocorrect the mobility, the drain current Ids is carried through the drivetransistor Trd in the state in which the gate G of the drive transistorTrd is fixed to the level of the video signal Vsig. Here, sinceVss1−Vth<VthEL is set to allow the light emitting device EL in thereverse bias state, simple capacitance characteristics are shown, notthe diode characteristics. Therefore, the current Ids carried throughthe drive transistor Trd is written to the capacitance C=Cs+Coled thatcombines the pixel capacitance Cs with the equivalent capacitance Coledof the light emitting device EL. Thus, the source potential (S) of thedrive transistor Trd is rising. In the timing chart shown in FIG. 4,this rise is denoted by Δ V. Since the rise Δ V is eventually subtractedfrom the voltage Vgs between the gate and the source held in the pixelcapacitance Cs, it means that negative feedback is applied. As describedabove, the negative feedback of the output current Ids of the drivetransistor Trd is similarly applied to the input voltage Vgs of thedrive transistor Trd, whereby the mobility μ can be corrected. Inaddition, the amount of negative feedback Δ V can be optimized byadjusting the time width t of the period from T6 to T7 to correct themobility.

At timing T7, the control signal WS is turned to the low level, and thesampling transistor Tr1 is turned off. Consequently, the gate G of thedrive transistor Trd is separated from the signal line SL. Since theapplication of the video signal Vsig is released, the gate potential (G)of the drive transistor Trd is allowed to rise, and it rises togetherwith the source potential (S). During this period, the voltage Vgsbetween the gate and the source held in the pixel capacitance Csmaintains the value of (Vsig−ΔV+Vth). In association with the rise ofthe source potential (S), the reverse bias state of the light emittingdevice EL is cancelled, and then the output current Ids flows toactually start the light emission of the light emitting device EL. Therelation between the drain current Ids and the gate voltage Vgs at thistime is given as Equation (5) below by substituting Vsig−Δ V+Vth intoVgs of Equation (1) for the transistor characteristics above.Ids=kμ(Vgs−Vth)2=kμ(Vsig−ΔV)2  (5)

In Equation (5), k=(1/2) (W/L)Cox. It is revealed from Equation (5) forthe characteristics that the term Vth is cancelled and the outputcurrent Ids supplied to the light emitting device EL does not depend onthe threshold voltage Vth of the drive transistor Trd. Basically, thedrain current Ids is determined by the signal voltage Vsig of the videosignal. In other words, the light emitting device EL is to emit light inaccordance with the brightness corresponding to the video signal Vsig.At this time, Vsig is corrected by the amount of feedback Δ V. Thecorrecting amount Δ V works so as to cancel the effect of the mobility μpositioned at the coefficient part of Equation (5) for thecharacteristics. Therefore, the drain current Ids substantially dependsonly on the video signal Vsig.

Finally, at timing T8, the control signal DS is turned to the highlevel, the switching transistor Tr4 is turned off, and light emission isfinished as well as the field is ended. After that, the process goes tothe subsequent field, and the Vth correction operation, the mobilitycorrecting operation and the light emission operation are againrepeated.

FIG. 9 shows a circuit diagram depicting the state of the pixel circuit2 in the period from T6 to T7 to correct the mobility. As shown in thedrawing, in the period from T6 to T7 to correct the mobility, thesampling transistor Tr1 and the switching transistor Tr4 are turned on,whereas the remaining switching transistors Tr2 and Tr3 are turned off.In this state, the source potential (S) of the drive transistor Tr4 isVss1−Vth. The source potential (S) is also the anode potential of thelight emitting device EL. As described above, Vss1−Vth<VthEL is set, andthus the light emitting device EL in the reverse bias state, and showssimple capacitance characteristics, not the diode characteristics.Therefore, the current Ids carried through the drive transistor Trdflows into the capacitance C=Cs+Coled that combines the pixelcapacitance Cs with the equivalent capacitance Coled of the lightemitting device EL. In other words, the negative feedback of a part ofthe drain current Ids is applied to the pixel capacitance Cs to correctthe mobility.

FIG. 10 shows a graph depicting Equation (5) for the transistorcharacteristics described above, in which the vertical axis indicatesIds, and the horizontal axis indicates Vsig. Below the graph, Equation(5) for the characteristics is also shown. The graph shown in FIG. 10depicts the characteristic curve as the pixel 1 is compared with thepixel 2. The mobility μ of the drive transistor of the pixel 1 isrelatively great. In contrast to this, the mobility μ of the drivetransistor included in the pixel 2 is relatively small. As describedabove, when the drive transistor is configured of a polysilicon thinfilm transistor, it is inevitable to vary the mobility μ among thepixels. For example, in the case in which the signal potential Vsig ofthe video signal at the same level is written to the pixels 1 and 2, ifthe mobility is not corrected at all, a large difference occurs betweenthe output current Ids1′ carried through the pixel 1 with a greatermobility μ and the output current Ids2′ carried through the pixel 2 witha smaller mobility μ. As described above, since a large differenceoccurs between the output currents Ids caused by the variations inmobility μ, unevenness like streaks occurs to impair the uniformity ofthe screen.

Then, in an embodiment of the invention, the negative feedback of theoutput current is applied to the input voltage side, whereby thevariations in mobility are cancelled. As apparent from Equation (1) forthe transistor characteristics, the greater mobility is, the larger thedrain current Ids is. Therefore, the amount of negative feedback ΔVbecomes larger as the mobility is greater. As shown in the graph in FIG.10, the amount of negative feedback ΔV1 of the pixel 1 with a larger themobility μ is greater than the amount of negative feedback ΔV2 of thepixel 2 with a smaller mobility. Therefore, a larger negative feedbackis applied as the mobility μ is greater, and then variations can besuppressed. As shown in the drawing, when ΔV1 is corrected in the pixel1 with a greater mobility μ, the output current greatly drops from Ids1′to Ids1. On the other hand, sine the correcting amount ΔV2 of the pixel2 with a smaller mobility μ is small, the output current Ids2′ does notso greatly drop to Ids2. Consequently, Ids1 is almost equal to Ids2, andthe variations in mobility are cancelled. Since the cancellation of thevariations in mobility is performed in the entire range of Vsig from theblack level to the white level, the uniformity of the screen issignificantly high. In summary, in the case in which there are thepixels 1 and 2 having different mobilities, the correcting amount ΔV1 ofthe pixel 1 with a greater mobility is smaller than the correctingamount ΔV2 of the pixel 2 with a smaller mobility. In other words, ΔV islarger as the mobility is greater, and the reduced value of Ids islarger. Therefore, the mobility of the current values of the pixelshaving different mobilities is made equal, and the variations inmobility can be corrected.

Hereinafter for reference, the numerical analysis of mobility correctiondescribed above will be described. As shown in FIG. 9, in the state inwhich the transistor Tr1 and Tr4 are turned on, a variable V is takenfor the source potential of the drive transistor Trd for analysis. Thedrain current Ids carried through the drive transistor Trd is asexpressed in Equation (6) below where the source potential (S) of thedrive transistor Trd is V.I _(ds) =Kμ(V _(gs) −V _(th))² =Kμ(V _(sig) −V−V _(th))²  (6)

In addition, from the relation between the drain current Ids and thecapacitance C(=Cs+Coled), Ids=dQ/dt=CdV/dt is held as shown in Equation(7) below.

$\begin{matrix}{{{{From}\mspace{14mu} I_{ds}} = {\frac{dQ}{dt} = {C\frac{dV}{dt}}}},{{\int{\frac{1}{C}{dt}}} = { {\int{\frac{1}{I_{ds}}{dV}}}\Leftrightarrow{\int_{0}^{t}{\frac{1}{C}{dt}}}  = { {\int_{- {Vth}}^{V}{\frac{1}{k\;{\mu( {V_{sig} - V_{th} - V} )}^{2}}{dV}}}\Leftrightarrow{\frac{k\;\mu}{C}t}  = {\lbrack \frac{1}{V_{sig} - V_{th} - V} \rbrack_{- {Vth}}^{V} = { {\frac{1}{V_{sig} - V_{th} - V} - \frac{1}{V_{sig}}}\Leftrightarrow{V_{sig} - V_{th} - V}  = {\frac{1}{\frac{1}{V_{sig}} + {\frac{k\;\mu}{C}t}} = \frac{V_{sig}}{1 + {V_{sig}\frac{k\;\mu}{C}t}}}}}}}}} & (7)\end{matrix}$

Equation (6) is substituted into Equation (7), and both sides areintegrated. Here, the initial state of the source voltage V is −Vth, andthe correcting time (T6 to T7) for mobility variations is t. When thisdifferential equation is solved, the pixel current with respect to thetime t to correct mobility is given by Equation (8) below.

$\begin{matrix}{I_{ds} = {k\;{\mu( \frac{V_{sig}}{1 + {V_{sig}\frac{k\;\mu}{C}t}} )}^{2}}} & (8)\end{matrix}$

The display device according to an embodiment of the invention has thedevice configuration of thin films as shown in FIG. 13. This drawingschematically shows the cross sectional structure of a pixel formed onan insulating substrate. As shown in the drawing, the pixel includes atransistor part including a plurality of thin film transistors (a singleTFT is shown in the drawing), a capacitance part such as holingcapacitance, and a light emitting part such as an organicelectroluminescent device. The transistor part and the capacitance partare formed on the substrate by TFT processes, and the light emittingpart such as the organic electroluminescent device is laminated thereon.A transparent counter substrate is bonded thereon with an adhesive toform a flat panel.

As shown in FIG. 14, the display device according to an embodiment ofthe invention may include a display device in a flat module shape. Forexample, on the insulating substrate, a pixel array part is arranged inwhich pixels formed of an organic electroluminescent device, thin filmtransistors, and a thin film capacitance are integrated and formed in amatrix. An adhesive is applied so as to surround the pixel array part(the pixel matrix part), and a counter substrate such as a glasssubstrate is bonded to form a display module. On the transparent countersubstrate, a color filter, a protective film, and a light shielding filmmay be disposed as necessary. For example, on the display module, a FPC(flexible printed circuit) may be disposed as a connector to externallyinput and output signals to the pixel array part.

The display device described above according to an embodiment of theinvention has a flat panel shape, and can be adapted to the displays ofelectronic appliances in various fields such as a digital camera, anotebook personal computer, a cellular telephone, and a video camera, inwhich video signals inputted to the electronic appliance or generated inthe electronic appliance are displayed as an image or video.Hereinafter, an exemplary electronic appliance is shown to which such adisplay device is adapted.

FIG. 15 shows a television set to which an embodiment of the inventionis adapted, including a video display screen 11 configured of a frontpanel 12, a filter glass 13 and so on, which is fabricated by using thedisplay device according to an embodiment of the invention for the videodisplay screen 11.

FIG. 16 shows a digital camera to which an embodiment of the inventionis adapted, the upper is a front view and the lower is a rear view. Thisdigital camera includes an imaging lens, a flash light emitting part 15,a display part 16, a control switch, a menu switch, a shutter 19, and soon, which is fabricated by using the display device according to anembodiment of the invention for the display part 16.

FIG. 17 shows a notebook personal computer to which an embodiment of theinvention is adapted, including a keyboard 21 that is operated whenletters are inputted to a main body 20, and a main cover includes adisplay part 22 that an image is displayed, which is fabricated by usingthe display device according to an embodiment of the invention for thedisplay part 22.

FIG. 18 shows a mobile terminal device to which an embodiment of theinvention is adapted, the left shows it opening, and the right shows itclosed. The mobile terminal device includes an upper housing 23, a lowerhousing 24, a joining part (here, a hinge part) 25, a display 26, asub-display 27, a picture light 28, a camera 29 and so on, which isfabricated by using the display device according to an embodiment of theinvention for the display 26 and the sub-display 27.

FIG. 19 shows a video camera to which an embodiment of the invention isadapted, including a main body 30, a lens 34 for shooting a subject onthe side surface facing to the front, a start/stop switch 35 whenshooting, a monitor 36, and so on, which is fabricated by using thedisplay device according to an embodiment of the invention for themonitor 36.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An image display device comprising: a pluralityof scanning lines arranged in rows; a plurality of signal lines arrangedin columns; a scanner part configured to supply a control signal to theplurality of scanning lines; a plurality of pixel circuits arranged in amatrix, respective ones of the plurality of pixel circuits connected toa corresponding one of the plurality of scanning lines and acorresponding one of the plurality of signal lines, wherein respectiveones of the plurality of pixel circuits include a pixel capacitor, asampling transistor, a drive transistor having a first channel length, afirst switching transistor having a second channel length, and a lightemitting device, and the first channel length is longer than the secondchannel length.
 2. The image display device according to claim 1,wherein respective ones of the plurality of pixel circuits furtherinclude a second switching transistor having a third channel length, andthe first channel length is longer than the third channel length.
 3. Theimage display device according to claim 2, wherein respective ones ofthe plurality of pixel circuits further include a third switchingtransistor having a fourth channel length, and the first channel lengthis longer than the fourth channel length.
 4. The image display deviceaccording to claim 3, wherein a gate of the sampling transistor isconnected to a first scanning line of the plurality of scanning lines, agate of the first switching transistor is connected to a second scanningline of the plurality of scanning lines, a gate of the second switchingtransistor is connected to a third scanning line of the plurality ofscanning lines, and a gate of the third switching transistor isconnected to a fourth scanning line of the plurality of scanning lines.5. The image display device according to claim 3, wherein the firstswitching transistor is configured to selectively supply a firstpredetermined potential to the pixel capacitor, the second switchingtransistor is configured to selectively supply a second predeterminedpotential to an electrode of the light emitting device, and the thirdswitching transistor is configured to selectively supply a thirdpredetermined potential to the drive transistor.
 6. The image displaydevice according to claim 1, wherein the scanner part includes a scannertransistor having a fifth channel length, and the first channel lengthis longer than the fifth channel length.
 7. The image display deviceaccording to claim 1, wherein the first channel length is at least 10μm.
 8. The image display device according to claim 7, wherein the firstchannel length is from 15 μm to 20 μm.
 9. The image display deviceaccording to claim 1, wherein the drive transistor is formed by a lowtemperature polysilicon process.
 10. The image display device accordingto claim 1, wherein the drive transistor and the sampling transistorrespectively include a low temperature polysilicon having a plurality ofgrains.
 11. An electronic appliance, comprising: an image display deviceincluding: a plurality of scanning lines arranged in rows, a pluralityof signal lines arranged in columns, a scanner part configured to supplya control signal to the plurality of scanning lines, a plurality ofpixel circuits arranged in a matrix, respective ones of the plurality ofpixel circuits connected to a corresponding one of the plurality ofscanning lines and a corresponding one of the plurality of signal lines,wherein respective ones of the plurality of pixel circuits include apixel capacitor, a sampling transistor, a drive transistor having afirst channel length, a first switching transistor having a secondchannel length, and a light emitting device, and the first channellength is longer than the second channel length.
 12. The electronicappliance according to claim 11, wherein respective ones of theplurality of pixel circuits further include a second switchingtransistor having a third channel length, and the first channel lengthis longer than the third channel length.
 13. The electronic applianceaccording to claim 12, wherein respective ones of the plurality of pixelcircuits further include a third switching transistor having a fourthchannel length, and the first channel length is longer than the fourthchannel length.
 14. The electronic appliance according to claim 13,wherein a gate of the sampling transistor is connected to a firstscanning line of the plurality of scanning lines, a gate of the firstswitching transistor is connected to a second scanning line of theplurality of scanning lines, a gate of the second switching transistoris connected to a third scanning line of the plurality of scanninglines, and a gate of the third switching transistor is connected to afourth scanning line of the plurality of scanning lines.
 15. Theelectronic appliance according to claim 13, wherein the first switchingtransistor is configured to selectively supply a first predeterminedpotential to the pixel capacitor, the second switching transistor isconfigured to selectively supply a second predetermined potential to anelectrode of the light emitting device, and the third switchingtransistor is configured to selectively supply a third predeterminedpotential to the drive transistor.
 16. The electronic applianceaccording to claim 11, wherein the scanner part includes a scannertransistor having a fifth channel length, and the first channel lengthis longer than the fifth channel length.
 17. The electronic applianceaccording to claim 11, wherein the first channel length is at least 10μm.
 18. The electronic appliance according to claim 17, wherein thefirst channel length is from 15 μm to 20 μm.
 19. The electronicappliance according to claim 11, wherein the drive transistor is formedby a low temperature polysilicon process.
 20. The electronic applianceaccording to claim 11, wherein the drive transistor and the samplingtransistor respectively include a low temperature polysilicon having aplurality of grains.